The present invention relates to a flame retarder having an excellent anti-blooming property, and more particularly to an anti-blooming flame retarder suited for use in imparting the flame retardancy to improved woods. The invention also relates to a method of application of the anti-blooming flame retarder.
In recent years, it is strongly required that house and building construction materials, interior finish materials for house, building and vehicles, furnitures, fittings and domestic electrified equipments and appliances are flame resistant, and various flame retardant agents are developed to satisfy the requirement. When treating plywoods employed as the materials for house, building, furnitures and fittings or fiber-boards employed as the materials for house, building, interior finish and domestic electrified equipments and appliances to render them flame retardant, water-soluble flame retardant agents are usually employed. However, the plywoods and fiber-boards treated with water-soluble flame retardant agents raise a problem that the water-soluble flame retardant agent contained within the plywoods or fiber-boards migrates to the surface with the lapse of time and causes the so-called blooming.
The blooming is caused mainly by the change in humidity of air with the lapse of time. That is to say, a water-soluble flame retardant agent contained in a woody material such as plywoods or fiber-boards is dissolved in or deliquesces by moisture, when the humidity of air is high and are in the wet state. Thereafter, when the humidity of air lowers, the moisture inside the material migrates to the surface and evaporates with the lowering of the humidity. At the same time, the water-soluble flame retardant agent which has been dissolved or deliquesced migrates from the inside of the material to the surface with the migration of the moisture, and appears in the form of crystal on the surface of the material when the mosture evaporates. This phenomenon takes place with every repetition of drying and wetting of air, and the flame retardant agent migrated to the surface is fixed and accumulated on the surface so that the surface presents an appearance as if it had a bloom. This phenomenon is called blooming.
Phosphorus-containing inorganic salts such as ammonium dihydrogenphosphate, diammonium hydrogenphosphate and condensed ammonium polyphosphate, bromine-containing inorganic salts such as ammonium bromide, boron-containing inorganic compounds such as sodium borate, potassium borate and boric acid, and aluminum-containing inorganic salts such as alum are generally employed as the water-soluble flame retardant agents for treating woody materials such as plywoods and fiber-boards. The solubility in water of these flame retardant agents is about 20 to about 60 g. to 100 g. of water at ordinary temperature, and sodium bromide, ammonium dihydrogenphosphate, diammonium hydrogenphosphate and condensed ammonium polyphosphate have the deliquescent property.
As a method for inhibiting or preventing the blooming, there are proposed (1) a method in which a material to be treated is impregnated with an aqueous solution of a water-soluble flame retardant agent and then heat-treated to render the water-soluble flame retardant agent water-resistant, and (2) a method in which a material is impregnated with an aqueous solution of a water-soluble flame retardant agent and the flame retardant agent is chemically bonded to the material to prevent the migration by moisture. However, the method (1) has the disadvantage of being hard to apply to most of the water-soluble flame retardant agents generally employed. That is to say, it is difficult to convert the usually employed inorganic salt flame retardant agents to water-resistant substances, even if heat-treated at the highest possible temperature within the range of a material to be treated not being deteriorated after impregnating the material therewith. The method (2) also has the disadvantage that applicable water-soluble flame retardant agents are limited to the above-mentioned phosphorus-containing inorganic salts which can undergo the esterification reaction with cellulose of the material to be treated. Moreover, even in the case of using the phosphorus-containing inorganic salts as flame retardant agents, it is difficult to bond the whole of the flame retardant agent contained in the material to cellulose of the material. Therefore, a fairly large amount of the flame retardant agent remains unreacted, and the occurrence of blooming due to the unreacted flame retardant agent is unavoidable.
Thus, the known methods of preventing the blooming are hard to apply to usual water-soluble flame retardant agents, or even if applicable, they cannot sufficiently prevent the blooming and, therefore, a flame retardant agent or a method of imparting flame retardancy, which can completely prevent the blooming, is strongly desired.